The invention relates to a device and a method for the inductive sealing of a plurality of plies of a laminate.
Laminates are frequently used as packaging material for packagings of liquid foodstuffs which comprise a carrier layer made of electrically non-conductive material, in particular paper or card, and a sealing layer made of thermoplastic material, in particular thermoplastic plastic such as for example polyethylene or polypropylene. The thermoplastic material is liquid-tight and resistant to fatty substances and acids. A metal layer, in particular an aluminium layer, is disposed between the sealing layer and the carrier layer. The thermoplastic material is heat-sealable by pressing against one another mutually facing sealing layers of a plurality of laminate plies and welding together by application of heat. The heat is produced by inductive sealing by installing at least one coil (inductor) in the sealing jaws for pressing together the plies of the laminate, which coil induces eddy currents in the metal layer and heats this. As a result of heat conduction, this heat is transferred from the heated metal layer onto the adjacent layers of thermoplastic material which enter into the molten state under the action of heat.
A device for thermal sealing of packaging materials is already known from CH 676958 A5, which has a metal layer in addition to a thermoplastic layer. The device comprises two compressible sealing jaws, where both are fitted with an inductor which is used for heating the metal layer by means of induction in order to weld the thermoplastic layers of two adjacent plies of the packaging material when pressing together the sealing jaws. The inductor comprises at least one elongate current conductor extending in the longitudinal direction of the sealing jaws, which is either embedded directly in an electrically non-conductive block of the sealing jaw or in an intermediate piece made of ferrite, where this intermediate piece is mounted in a block made of an electrically non-conductive material.
Short heating times are required when sealing gable packages for liquid foodstuffs in a filling machine. This is because a long pressing and cooling phase is used because a high tension is present in the geometry of the gable, which gives the gable the tendency to re-open after the pressing time has ended. This tendency can be countered by a sufficiently long cooling of the molten thermoplastic material. The required short heating times are achieved by a high energy input into the inductor per unit time. As a result however, the losses are increased which can lead to an inadmissible heating of the inductor and optionally to local overheating of the laminate.
Although the inductor follows the geometry of the regions of the gable package to be sealed, in practice it has been found that in some cases a non-uniform heating of the sealing layer occurs with the consequence that the sealing of the gable package is not perfect.
Starting from this prior art, it is the object of the invention to provide a device for the inductive sealing of a plurality of plies of a laminate which, taking into account the required short heating times and the associated high energy input per unit time, avoids a non-uniform heating of the sealing layer of the laminate and therefore non-perfect seals.
In addition, a method for inductive sealing is to be provided which in particular is suitable for integration into a filling system for liquid foodstuffs with an aseptic working area.